Abstract: The stigmas of five species, Populus euphratica Oliv., P. alba L., P. sirnonii Carr., P. lasiocarpa Oliv. and P. nigra L. have been studied. Scanning electron microscopy reveals that exudates are present in the intercellular spaces, in the clefts between the multicellular papillae and on the receptive surface. Release and movement of exudates can be visualized when the fresh stigmas are stained with sudan Ⅲ and auramine O. Paraffin and semithin resin sections of stigmas after glutaraldehyde-osmium fixation evidence the lipidic nature of the exudates. Transmission electron microscopy reveals the glandular features of the stigmatic papillae cells, such as abundance of rough and smooth endoplasmic reticulum, polyribosomes, and well-developed dictyosomes with secretory vesicles. Peuicle and epicuticular lamellate layers which have been considered as typical features of the dry-type stigmas are also present in the species where stigmas appear extremely wet. It is concluded that stigmas in all of the five species are secretory at the receptive stage. Well-developed generative and sperm ceils were observed in the pollen tubes penetrating through the deep layers of the stigmatic tissue in the reciprocal crosses between P. euphratica and P. simonii, which indicated that there is no significant barrier in the stigma.

Key words: Secretory stigma, Populus, Ultrastructure, Cytochemistry, Incompatibility